Heretofore, many solid state electronic devices have been shown to exhibit negative differential conductivity. Perhaps the first of such devices to be developed and understood is now commonly termed the Gunn diode. By negative differential resistance, it is meant that the current decreases as the field increases. This effect is important in microwave applications, such as in local oscillators or power amplifiers. Negative differential resistance arises in semiconductors which have two conduction bands, those with a lower valley and a higher mobility and a low mobility and high valley in the conduction band. The two minima are commonly referred to as the lower and upper valleys, respectively. As the electric field within the bulk semiconductor is increased, a point is reached at which some electrons are scattered from the high mobility, lower valley to the low mobility, upper valley and, as a result, the average carrier drift velocity is reduced as the electric field is increased after scattering to the upper valley commences. Hence, the current through the semiconductor decreases after this point because of the decrease in average carrier mobility even with the voltage increase. The negative differential resistance leads to, for example, the generation of coherent microwave radiation at a frequency between 1 GHz and 100 GHz. See U.S. Pat. No. 4,903,092, issued to Luryi et al on Feb. 20, 1990 which is incorporated by reference hereto.
At a sufficiently high bias, these devices switch from one conducting state to another, and under appropriate bias can be made to oscillate between the conducting states. It has been demonstrated that a heterostructure hot-electron diode in which the current voltage characteristics of single and multiple GaAs/AlGaAs barriers have an S-shaped negative differential conductivity (SNDC) at low temperatures. Hess et al, Journal of Applied Physics 60, pg. 3776, 1986. It has also been demonstrated that the instability of this device is the result of a change in the conduction mechanism from low bias tunneling through the AlGaAs barrier to high bias thermionic emission of hot-electrons over the barrier. Emanuel et al, Solid State Electronics 31, pg. 589, 1988. The interest, of course, in these devices has been due to their potential application as oscillators since they have very short switching times.